Recently, liquid crystal display (LCD) devices have been increasingly demanded as thin high-definition displays for use in computers, word processors, etc. Active-matrix-type LCD devices are, among others, widely used because of their high performance. Further, among the active-matrix-type LCD devices, LCD devices using thin film transistors (hereinafter referred to as TFTS) as switching elements are particularly in great use, for properties thereof.
FIG. 17 conceptually illustrates an arrangement of an LCD device of prior art. In the conventional LCD device, first and second glass substrates are provided to face each other with a certain gap therebetween, in which a liquid crystal layer is held. The first glass substrate is provided with gate bus lines 101, source bus lines 102 which cross the gate bus lines 101, and TFTs 103 each disposed in the vicinity of each of intersections of the gate bus lines 101 and the source bus lines 102. Each TFT 103 is connected with a corresponding pixel electrode 104. On the other hand, the second glass substrate is provided with color filters (not shown) at positions corresponding to the pixel electrodes 104, and a counter electrode 105.
In such a conventional LCD device, however, since the gate bus lines and the source bus lines are provided on the same substrate though being separated by an insulating film provided therebetween, defects such as breaking of bus lines at intersections of bus lines where one climbs over another, and short-circuiting of bus lines due to defects of the insulating film occur, resulting in poor yield in manufacture.
As solution of the foregoing problems, therefore, an arrangement wherein the gate bus lines are formed on the first glass substrate while the source bus lines are formed on the second glass substrate (hereinafter referred to as counter source structure) has been proposed by the following references (a) through (c), for example.
(a) "New Electrodes Architectures for Liquid Crystal Displays Based on Thin Film Transistors," J. F. Clerc et al., Japan Display '86.
(b) "A New Active Matrix LCD Architecture for Larger Size Flat Display" Kenichi Oki et al., ITEJ Technical Report Vol.11 No.27, pp.73-78.
(c) The Japanese Publication for Laid-Open Patent Application No.133478/1987 (Tokukaisho 62-133478, date of publication: Jun. 16, 1987).
In an LCD device of the counter source structure, a first glass substrate is provided with gate bus lines, reference signal lines for applying a reference voltage to a liquid crystal layer, pixel electrodes, and TFTs, while a second glass substrate is provided with source bus lines. The first and second glass substrates are arranged so as to face each other with a certain gap therebetween, in which liquid crystal material is held. With the counter source structure, since the gate bus lines and the source bus lines thus do not intersect on one and same substrate, the lowering of yield in manufacture can be suppressed.
In order that color displaying be performed by an LCD device, it is necessary to arrange, on the second glass substrate, filters selectively transmitting waves with specific wavelengths. Besides, to enhance the display performance, in the normally white mode of an LCD using twist nematic liquid crystal material, for example, the white display (when no electric field is applied) has transmissivity greatly different from that of the black display (when an electric field is applied), or to state differently, a higher contrast ratio has to be obtained. Accordingly, a light blocking film for preventing light leakage is formed on the second glass substrate. As the light blocking film, a black matrix made of metal material such as chromium (Cr) has conventionally been formed.
FIGS. 18(a) through 18(d) are plan views and cross-sectional views illustrating an example of a manufacturing process of the second glass substrate. First, Cr is deposited on a glass substrate 111 by sputtering, which is followed by predetermined patterning through photolithography, etching, and resist-removing washing steps. As a result, a black matrix 112 is formed (see FIG. 18(a)).
Subsequently, a resin film (dry film) in which red pigment is dispersed is laminated thereon (the surface is coated with the resin film), then subjected to exposure, development, and baking, resulting in that a red (R) pattern 113 is formed. Further, a resin film in which green pigment is dispersed is laminated thereon, and a green (G) pattern 114 is formed in an identical manner to that for the red (R) pattern 113. Further, the blue (B) pattern 115 is formed in an identical manner to those for the red (R) pattern 113 and the green (G) pattern 114. Thus, three color layers are formed (see FIG. 18(b)). Incidentally, the same color filter formation process as that described above has been applied to the conventional LCD devices in which gate bus lines and source bus lines are both formed on the first glass substrate, as well as to the LCD devices of the counter source structure in which gate bus lines are formed on the first glass substrate while source bus lines are formed on the second glass substrate.
Next, a resin material is applied by spin coating over an entire surface of the glass substrate 111 on which the color filters are formed, and is subjected to baking, so that a transparent resin film (overcoat) 116 is formed (see FIG. 18(c)).
Then, an ITO film (indium tin oxide) is formed by sputtering, followed by predetermined patterning through photolithography, etching, and resist-removing washing steps. As a result, source bus lines 117 are formed (see FIG. 18(d))
In the LCD device of the counter source structure, however, gate signal input terminals and source signal input terminals are formed on respective different substrates. Besides, in the case of the counter source structure, a wire material of gate bus lines and that of source bus lines are generally different from each other. Since the gate bus lines and the source bus lines differ in film composition, the gate signal input side and the source signal input side differ in film thickness; This results in that a sealing member to be placed between the substrates for making the substrates adhere to each other and holding a liquid crystal material between the substrates is provided between surfaces with projections and recesses which are different on the gate signal input side and the source signal input side. Further, as thickness of the film on the substrate varies from the signal input side to the signal non-input side, it is difficult to keep the gap between the substrates uniform all along the four edges at-which the sealing material is applied, thereby causing a drawback in that the cell thickness of the liquid crystal panel becomes non-uniform.
Thus, the non-uniformity of cell thickness of the liquid crystal panel caused by projections and recesses due to the bus lines is remarkable particularly in the case of an LCD device of the counter source structure.
Generally, for an LCD device, to keep the distance between the two substrates uniform is important with view to good display performance, and this is more remarkable as the LCD device has a greater screen size.